Sleep and its timing - longitudinal and cross-sectional associations with cognitive performance and health in youth

Sleep has important functions for both health and cognitive performance. This thesis investigated typical, non-restricted sleep over a developmental span from middle childhood to early adulthood. Specifically, the focus was (1) identifying the cross-sectional associations between neurocognitive functioning and habitual sleep duration and sleep quality in early adolescence, and (2) studying the sleep behaviour associated with young adults’ executive functioning and self-control. Additionally, the goals included (3) studying the longitudinal associations between naturally occurring sleep and lipid profile in early adolescence, and, finally, (4) differentiating the developmental trajectories of sleep timing from middle childhood to adolescence from a circadian preference perspective. The participants came from two population-based cohorts, and all the studies in this thesis were done using actigraphs with piezoelectric accelerometers, which provide objective sleep measures based on movement. We found that during early adolescence, especially boys’ shorter sleep duration was associated with poorer performance in tests evaluating executive functioning. We found similar results in young adults, but also found that later sleep timing and regularity were associated with weaker trait-like executive functioning, such as self-control and behaviour regulation. Longitudinal analyses revealed that girls’ shorter sleep duration and irregular sleep in middle childhood were associated with a more detrimental lipid profile (higher levels of LDL cholesterol and triglycerides, lower levels of HDL cholesterol) in later life, during early adolescence. These associations survived controlling for body mass index and physical activity. We also analysed the sleep patterns of different circadian preference phenotypes longitudinally, and found that those adolescents with a preference for morningness differed from others in sleep timing already at age 8. This suggests long-term stability in sleep patterns. Based on these findings, objectively measured sleep and its timing have longitudinal pathways which connect to future health, and may act as risk factors or as protective features for various health related outcomes. It is also likely that sleep, self-control, and health behaviour are intertwined during development.